In the manufacture of so-called integrated circuits, an image from a mask is generally transferred to a wafer having photoresist material thereon. The photoresist material on the wafer reacts to light projected thereon to cause an image to be reproduced.
In some cases, the mask is laid against the wafer to produce the image. However, in a patent "Optical Projection and Scanning Apparatus," U.S. Pat. No. 4,068,947, issued Jan. 17, 1978, there is disclosed a system wherein the mask is not in contact with the wafer to reproduce the image. In this patent light is projected through the mask into an imaging system and then onto the wafer to image the mask onto the wafer thereby, increasing the life of the mask since no friction is involved between the two elements. An optical system is disposed between the mask and the wafer.
In the aforementioned patent, flat, convex and concave mirrors provide distortion free unit magnification with accessible object and image planes in which the contrast is uniformly high and relatively constant.
In order to obtain best results, the illumination on the mask reproduced is a curved arc of illumination that conforms to a narrow zone of good correction concentric with the axis of the projection optical system. The illuminated area on the mask is determined by an illumination system (condenser) that contains a curved slit that is imaged on the mask plane of the projection system where the optical correction of the projection system is optimum.
Means for scanning and various optical elements of the system are described in the aforementioned patent. Further details of the theory relating to the optical system involved is disclosed in another patent entitled "Unit Power Imaging Catoptric Anastigmat," U.S. Pat. No. 3,748,015, issued July 24, 1973. Both of the above mentioned patents are assigned to the same assignee as the present invention.
While the above system operates very satisfactorily, it was found that in some cases the illumination or intensity of the light transmitted through the slit opening to the wafer sometimes tends to vary slightly along the length of the slit opening. In order to assure optimum image reproduction, it is desirable that the light along the entire length of the slit opening be uniform. The present invention is directed generally towards a system illustrated in FIG. 5 of U.S. Pat. No. 4,068,947, although clearly not limited to such systems. Particularly, it is directed towards slit element 512 in FIG. 5.